Researchers released the early findings Tuesday, just weeks before the shots are set to begin much larger testing to prove if they're really strong enough to protect against the coronavirus.
The vaccine is made by the National Institutes of Health and Moderna Inc. It's one of nearly two dozen possible COVID-19 vaccines in various stages of human testing around the world.
The first U.S. study was small, just 45 people, and researchers found more than half experienced brief, flu-like reactions to the shots. The next step: Recruiting 30,000 people for the final testing.
VIDEO: Here's what goes into creating vaccine for COVID-19